1. Field of the Invention
The present invention generally relates to a gain-control method and device for cascaded programmable gain amplifiers and, more specifically, to a gain-control method and device suitable for controlling at a high speed the gain of cascaded programmable gain amplifiers in an integrated circuit (IC) for radio receivers.
2. Discussion of Background
In a radio receiver, the signal level inputted from the antenna covers a very wide range, which is converted into a suitable level by cascaded programmable gain amplifiers (PGAs) and is finally controlled so as to swing to the maximum within the input dynamic range of the ADC.
The PGAs are located on the pre-stages of band pass filters (BPFs) because the BPFs are apt to generate high noises. Taking on this configuration will effectively enhance the signal to noise ratio.
Gain control of cascaded PGAs may be carried out in the following manner. An ADC detects the final signal level experienced by the PGAs. On the basis of the detected result, the control and operation device determines new gains, and feeds back the gains to the PGAs each to thereby switch the gains of each of the PGAs simultaneously or sequentially.
The initial gains of the PGAs are set to voluntary values, however the signal level before a preamble term is usually low, and so, the gains of the PGAs are set to the maximum, for example, +20 dB as the initial setting. Until the control and operation device of the radio receiver determines that the signal level entering the ADC becomes the maximum in the input dynamic range of the ADC, the gain control is repeated in succession.
Repetitive gain controls are required several times to determine the final optimum gain. At each repetitive control, the method has to ensure the waiting time for awaiting the response by the band-pass filters. Unfortunately, it takes considerable time for the outputs of the cascaded amplifiers to reach convergence (or stabilization). In practice, there are many cases where convergence cannot be obtained within a required time period.